Add unit tests for regnum2phyx

[gaurav]

regnum2phyx.js is our script for converting Regnum JSON dumps into Phyx files. We should test this process to make sure it is working correctly by testing its outputs using JSON Schema, SHACL or ShEx to do shape-based validation. This would be a good thing to have for Phyx as we publish it anyway (phyloref/phyx.js#34), so doing this would solve several problems at once.

Another way to test whether regnum2phyx is working correctly would be to see if I can resolve all those phyloreferences using the Open Tree Resolver.